This invention relates to apparatus for measuring the distance between a metallic body, for example a steel material and a sensor of a measuring apparatus and more particularly to a method for measuring the distance between a detecting coil and a metallic body at high accuracies without contacting the metallic body.
For the measurement of a metallic body, for example measurement of the bend, position and so on of a steel plate during its rolling process, at high accuracies without contacting the metallic body, a method utilizing electromagnetic induction as diagramatically shown in FIG. 1 has been used. In the figure, an object (metallic body) to be measured is indicated by the numeral 1. A reference oscillator 2 generating AC signals of a predetermined frequency is connected to the input terminals of an AC brige circuit 3 consisting of impedances Z.sub.1, Z.sub.3 and Z.sub.4 having known impedances and an impedance Z.sub.2, the impedance Z.sub.2 being variable and, in the figure, represented by the impedance of a detecting coil 4. A differential amplifier 5 is connected to the output terminals of the bridge circuit 3. With the distance between the detecting coil 4 and the metallic body 1 being substantially infinite, and the AC signals from the reference oscillator 2 being applied to the bridge circuit 3 including the detecting coil 4, the bridge circuit becomes balanced when the condition Z.sub.1.Z.sub.4 =Z.sub.2.Z.sub.3 is satisfied. Under such balanced condition, the output from the bridge circuit 3 is zero and hence the differential amplifier 5 produces no output. As the distance D between the detecting coil 4 and the metallic body 1 decreases from the aforesaid infinite stage, the impedance Z.sub.2 of the detecting coil 4 varies with the distance D due to the variation of the self-induction caused by the electromagnetic induction in the metallic body. As is well known in the art the impedance Z.sub.2 varies non-linearly with respect to the distance D. As a result, the variation of the output of the brige circuit 3 is also non-linear. The output of the brige circuit 3 is amplified by the differential amplifier 5 to a prescribed value and then supplied to an indicating meter or a recording meter, and thus the measurement of the distance D can be carried out without contacting the metallic body. FIG. 2 shows an example of a relational characteristic between the distance D and the output voltage V.sub.pp of the differntial amplifier.
As seen from FIG. 2, the output characteristic with respect to distance in such prior art measuring apparatus is non-linear. Especially, as the distance D increases the rate of output value variation decreases, and accordingly high accuracies cannot be attained. Consequently, it is necessary for practical use to linearize the characteristic by some means such as by providing a suitable external circuit. Moreover, as the rate of change of the impedance Z.sub.2 of the detecting coil with respect to the variation of the distance D between the detecting coil and the metallic body is very small, the bridge circuit 3 is needed for obtaining sufficiently high sensitivity. The accuracy of measurement is greatly influenced by the accuracy and characteristics of the fixed impedances Z.sub.1, Z.sub.3 and Z.sub.4 constituting the bridge circuit, and high skill is required for adjusting the bridge circuit.
For overcoming such disadvantageous problems in the prior art, the inventors presented a method and apparatus as shown by FIG. 3 (Ser. No. 521,812, now U.S. Pat. No. 3,997,835). In the figure, similar numerals indicates like parts as in FIG. 1 having like effects. A feedback amplifier 6 is connected to a reference oscillator 2 through a series resistor R.sub.s. A feedback resistor R.sub.p is connected in parallel to the amplifier 6. A capacitor C is connected in parallel to the detecting coil 4 to form a parallel resonance circuit together with the impedance of the detecting coil 4. AC signals of a fixed amplitude and frequency from the reference oscillator 2 are applied to the parallel resonance circuit through the series resistor R.sub.s, and the voltage across the parallel resonance circuit is applied to the input terminal of the feedback amplifier 6. The feedback amplifier 6 amplifies the input voltage to an output voltage of a desired value which is then positive-feedbacked to the input side of the amplifier 6 through the feedback resistor R.sub.p, thus constituting a Q-multiplier.
Now, the distance between the detecting coil 4 and a metallic body 1 is set to be substantially infinite, and the capacitance of the capacitor C is so adjusted that the resonance frequency of the parallel resonance circuit becomes equal to the resonance frequency of the oscillator 2.
Under such conditions, when the distance between the detecting coil 4 and the metallic body 1 decreases, the alternating magnetic field generated by the detecting coil 4 interlinks with the metallic body 1 to change the impedance of the detecting coil 4. As a result, an input voltage corresponding to the distance between the metallic body 1 and the detecting coil 4 is applied to the feedback amplifier 6 and then is amplified to be an output voltage at the output terminal of the amplifier 6. Thus, by measuring this output voltage, the distance between the metallic body 1 and the detecting coil 4 can be determined with high accuracy.
The aforesaid problems can be overcome by the apparatus of FIG. 3, but other disadvantageous problems such as the following appear with this apparatus:
(1) as the measurement accuracy is influenced by the variation in the oscillation frequency of the reference oscillator 2, a high stability oscillator such as an X-tal oscillator is needed for practical use;
(2) The impedance of the detecting coil 4 changes in accordance with the temperature change to make the resonance frequency of the parallel resonance circuit vary causing a decreased measurement accuracy;
(3) the oscillation frequency of the reference oscillator and the oscillation frequency of the parallel resonance circuit must be equalized to each other, but the adjustment for the equalization is not easy; and
(4) as the resonance circuit is employed, the frequency characteristic is not good due to the band characteristic.